This invention relates to communications tuning systems, and more particularly, to heterodyne communication tuning systems.
The width of a radio frequency (RF) bandpass filter must be narrow enough to reject local oscillator (LO) frequencies when a heterodyne communication system is operating in a transmitting mode, and to reject image frequencies when the communication system is operating in a receiving mode. To generate the transmit signal, the local oscillator signal is coupled into the mixer along with the intermediate frequency (IF) signal and mixed to produce the desired RF frequency. However the mixer produces not only the desired RF signal, but also passes with some degree of attenuation, the local oscillator signal. Design specifications placed on transmitters require the local oscillator frequency signal be attenuated such that the desired RF signal is many dB, typically on the order of 40 dB, greater in signal strength than undesired frequencies, in this case the local oscillator frequency. In the past, the usual method of attenuating the local oscillator signal has been to pass the signal through a RF bandpass filter which is narrow enough to pass only the desired RF signal. Since the difference between the local oscillator frequency and the desired RF signal is the IF frequency, the bandpass characteristics of the bandpass filter are necessarily narrow with usually steep side slopes. This problem is compounded in a single conversion general coverage transmitter since the output frequency is one of a range of frequencies and the single conversion produces an output RF signal which is fairly close to the local oscillator frequency, for example on the order of 30 megahertz. A solution in the past has been to use a plurality of discrete bandpass filters which must be overlapping yet narrow enough to reject the LO frequency. Thus the filter bandwidth must be no greater than the difference between the desired RF signal and the local oscillator frequency which, as stated before, is equal to the frequency of the IF stage.
In a communications receiver, the signal which must be rejected by the RF bandpass filter is the image frequency. The image frequency is that signal having a frequency which is converted to the IF frequency during mixing with the local oscillator signal. Since the mixer produces both the sum and the difference between a local oscillator and the RF signal, both RF signals lying on either side, that is frequencies greater than the LO frequency and less than the LO frequency by a difference equal to the If, will be converted in the mixer to the IF and be amplified by the IF stage. Thus it is necessary for the RF bandpass filter to reject the undesired image frequency. The rejection of this image frequency is completely analogous to the rejection of the LO frequency of a transmitter. The same solutions have been used in the past and although the image frequency differs from the desired RF frequency by twice the IF frequency, the multiple relatively narrow bandpass filters are still a relatively expensive part of a communications receiver.
Therefore it can be appreciated that a communications system which effectively suppresses local oscillator frequencies during transmitting and image frequencies during receiving without requiring a large number of narrow-band bandpass filters is highly desirable.